As an auxiliary light source for a liquid crystal display device used for a cellular phone or the like, a side-light type planar illumination device in which a primary light source is arranged on a side end face of a light guide plate is mainly used (hereinafter the side end face on which the primary light source is arranged is also called as a incoming-light face). As the primary light source of a side-light-type planar illumination device, a cold-cathode tube lamp has been used, but currently, a point-like light source such as a white LED which is more excellent in handling, easier in size reduction and better in impact resistance than the cold-cathode tube lamp is used in many cases. Application fields of the planar illumination device using such a point-like light source tend to expand, and application not only a small-sized liquid crystal display device used in a cellular phone or the like but also as an auxiliary light source of a liquid crystal display device used in an in-vehicle navigator with a relatively large display size, for example, is being discussed.
In order to respond to the expansion of an illumination area, various trials are made for increase of an outgoing light amount from the primary light source and efficient use of the outgoing light. For example, the increase of the outgoing light amount is promoted through increase of the number of point-like light sources arranged on one side end face of a light guide plate, arrangement of the point-like light source on a plurality of side end faces of the light guide plate or increase of the outgoing light amount per point-like light source by increasing an electric current to be supplied to the point-like light source.
On the other hand, increase of the point-like light sources constituting the primary light source or increase of the electric current to be supplied to each of the point-like light sources accompanies a problem of increase in a heat amount generated from the primary light source, leading to rise of an ambient temperature and lowered light emitting efficiency of the point-like light source. Also, when heat from the primary light source is transmitted to the light guide plate, positional displacement between the light guide plate and the point-like light source is induced by expansion of the light guide plate, which results in a problem that efficient use of the outgoing light is prevented and also causes fluctuation in illumination brightness of the planar illumination device. Moreover, if the light guide plate is accommodated in a frame, such nonconformity might occur that the light guide plate is deformed due to a difference in heat expansion coefficient between the light guide plate and the frame.
In order to efficiently radiate heat generated from the point-like light source, such a planar illumination device is proposed in which the light guide plate and the point-like light source are accommodated in a metal chassis and radiation performance of the point-like light source is improved by bringing the point-like light source into direct contact with the metal chassis (See Patent Document 1, for example). Also, a planar illumination device 100 in which an elastic action portion 120 is provided at a frame 102 as a method for restraining nonconformity caused by the heat expansion of the light guide plate shown in FIG. 3 is proposed (See Patent Document 2, for example).
In the planar illumination device 100, the elastic action portion 120 comprises a thin beam portion 122 capable of elastic deformation formed by providing a thickness reducing hole 123 on a side wall 102b of the frame 102 and a projection portion 121 projecting from the center of the beam portion 122. The frame 102 accommodates a light guide plate 103 and a fluorescent lamp 105 held by a lamp holder 104, and the light guide plate 103 is arranged so that the projection portion 121 of the elastic action portion 120 is brought into contact with a side end face (terminal face) 103b on the side opposite a side end face (incoming-light face) 103a on which the fluorescent lamp 105 is arranged. By holding the lamp holder 104 between the incoming-light face 103a and the side wall 102a of the frame 102 opposite the incoming-light face 103a, the fluorescent lamp 105 is arranged along the incoming-light face 103a of the light guide plate 103. In the planar illumination device 100, with the above configuration, the light guide plate 103 is energized to the fluorescent lamp 105 side with an elastic force F from the elastic action portion 120 and the deformation of the light guide plate 103 is absorbed.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-186004 (paragraphs [0035] to [0037], FIG. 3)
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-338214 (paragraphs [0016] to [0017], FIG. 1)